This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Shortly following G-CSF administration there is a dramatic yet transient disappearance of neutrophils from the circulation. The transient loss of neutrophils from the circulation following G-CSF administration appeared to be due to an accumulation of neutrophils within the pulmonary and splenic tissues. Detailed confocal microscopy using a myeloid/histiocyte antigen clone Mac387 which labels neutrophils and monocytes, an endothelial marker anti-caveolin-1-Cy3 antibody, and a nuclear stain (To-Pro3) showed that the neutrophils were elongated, appeared activated, and were tightly adhered to the endothelium of the pulmonary vasculature. These results suggest that the transient neutropenia following G-CSF administration is associated with an accumulation of neutrophils within the pulmonary and splenic tissues.